Democratic candidates in New York City’s primaries this year are energized and growing in numbers. Their targets aren’t Republican adversaries, they’re veteran Democratic incumbents.

In Brooklyn, 30 year-old Adem Bunkeddeko, a recent Harvard Business School graduate, is taking on Rep. Yvette Clarke, who has held her House seat for 11 years. In Queens, Alexandria Ocasio-Cortez, a 28-year-old former Bernie Sanders volunteer, is challenging Rep. Joseph Crowley, who is 55 years old and chair of the House Democratic Caucus. And...